Leonid Brezhnev headed to State Dacha Number 1 in Yalta, on the Crimean peninsula, just as he did every summer. It was August 13, 1968, and the Soviet leader was faced with a decision. Should he send tanks and soldiers to Czechoslovakia, because the comrades there were acting up, or should he give them one more chance?

The Communist Party in Prague had declared "democratic socialism" in the spring, upsetting the leaders of the other Warsaw Pact nations. Hard liners in Moscow were pushing for a military strike against the renegade reformers. But according to newly discovered documents, Brezhnev hesitated for a long time before finally ordering in troops on the night of August 21.

The decision-making process that led to the invasion can be reconstructed through documents SPIEGEL has recently gained access to. The documents are being published this week in a two-volume book by an international team of historians.

Leaders in Moscow first looked on complacently when reformer Alexander Dubcek replaced Stalinist Antonin Novotny as leader of Czechoslovakia's Communist Party. The new head had spent 13 years of his childhood and youth in the Soviet Union. It wasn't until later that it dawned on the Kremlin that Dubcek might want to go his own way. On March 25, the Soviet Central Committee warned the rest of the Communist bloc that Czechoslovakia was on the verge of turning into a "bourgeois republic."

In the summer, the conflict between Moscow and Prague intensified. On July 17, Brezhnev savaged the "excessive debates" of the Czechoslovak comrades. His opinion of Dubcek remained ambivalent: He was "not bad in front of the workers." But Brezhnev cursed the "greenhouse atmosphere for the weeds of the counterrevolution" in Czechoslovakia. He continued to urge patience: "Before we turn to extreme measures," he said, all other means should be tried "to bring the anti-socialist forces around by political means."

On August 13, Brezhnev called Dubcek from Yalta. He encouraged "Sasha," as he called the Czechoslovak leader, to fire three of the most radical reformers. Dubcek was evasive, promising nothing -- and was vague when Brezhnev demanded he take a hard line with the media. Dubcek offered his resignation: "I can also work somewhere else," he told the Soviet leader. Brezhnev was annoyed, and told Dubcek he wanted to avoid drastic measures.

By vacillating between threats ("We can't wait long") and conciliatory gestures ("Sasha, I believe you, but you have to understand my position") Brezhnev wanted to show that he was under pressure from his own party leadership. He couldn't after all "make a decision behind the backs of my Politburo members." The conversation ended coolly.

As proof that the "foundations of a socialist system had been shattered" in Prague, the Moscow Politburo could also point to a SPIEGEL interview with Justice Minister Bohuslav Kucera. In the interview, Kucera had called for independence for the judiciary and even raised the prospect of the Communist Party being replaced by another party in an election.

At 4 a.m. on August 21, Soviet paratroopers surrounded the building of the Communist Party's central committee on the Vltava River and stormed Dubcek's office. Soviet KGB officers then arrested the party leader. The tragedy had begun.

Brezhnev was shocked when he heard about the massive resistance in Prague and the provinces: There were dozens dead and hundreds injured. On August 23 he asked Czechoslovakia's President Ludvik Svoboda, who had travelled to Moscow, to figure out with him "how a massive bloodshed could be prevented."

Svoboda had his son-in-law, Milan Klusak -- the top civil servant of the Prague Foreign Ministry -- in tow. Klusak, who had been imprisoned by the Gestapo in 1942, said outright that in Prague people were "completely convinced" that East German soldiers had marched into the country alongside Soviet ones. Brezhnev, however, reassured him that was not the case. "Just between us, the German comrades were offended that we didn't trust them," he said.

In fact, Moscow and the leadership in Communist East Berlin had agreed that no combat units of the East German National People's Army should be deployed into the neighboring country.